This application relates to optical data storage, and more particularly, to optical near-field data storage.
Various optical storage devices and systems have been developed. One type of optical storage systems use an optical head to focus a monochromatic optical beam to a small spot on a recording layer of a storage medium for reading or writing data. The optical head may be positioned over the medium by a spacing greater than one wavelength, i.e., in a “far-field” optical configuration, where the optical energy is coupled between the optical head and the medium by light propagation. An optical head with a large numerical aperture can produce a small spot size. The diffraction effect in light propagation, however, limits the numerical aperture to less than unity. The areal density of such an optical storage device, hence, is limited by this diffraction-limited spot size which has a lower limit on the order of one half wavelength.
An optical storage system may also be configured to operate in a “near-field” configuration to achieve an areal density for data storage higher than that of the far-field configuration. In a typical near-field configuration, the optical head is spaced from the optical medium by a distance on the order of or less than one wavelength of the optical energy. The optical coupling between the optical head and the medium, therefore, can be effectuated at least in part by evanescent coupling, with or without light propagation. Some near-field devices couple optical energy through both evanescent coupling and coupling through light propagation. An effective numerical aperture of the optical head in such a near-field configuration can be greater than unity. Hence, a near-field optical storage system can achieve a focused beam spot size much less than one half wavelength and to realize a high areal storage density.
An optical head of an optical storage device or system generally includes an optical interfacing surface through which optical energy is coupled between the optical storage medium and the optical head. A near-field optical storage device may be designed in a “first surface” recording configuration, where the optical storage medium is designed to have the reflective layer formed between the recording layer and the substrate. During reading or writing operation, the optical interfacing surface of the optical head and the recording layer of the medium are located on the same side of the substrate of the medium. Hence, the optical beam is coupled from the optical head to a recording layer, or coupled from the recording layer to the optical head by reflection of the reflective layer, without passing through the substrate that supports the reflective layer, the recording layer, and other layers. The See, e.g., U.S. Pat. No. 6,243,350 to Knight et al. Thus, the substrate may not optically transparent. Because the near-field condition requires the optical interfacing surface of the head to be spaced from the surface of the optical medium by less than one wavelength, the optical head for the near-field first surface recording is designed to focus the optical beam essentially at or near the medium surface to achieve the minimum beam size in the recording layer.